1. Field of the Invention
The present invention relates to a brightness signal/color signal separating filter for separating a brightness signal and a color signal from a composite TV signal of, for example, an NTSC system.
2. Description of the Related Art
FIG. 12 is a block diagram of a conventional brightness signal/color signal separating filter of an NTSC system.
The operation of the conventional filter will be explained with reference to FIG. 12.
A composite color TV signal 5101 of an NTSC system input from an input terminal 5001 is converted into a digital signal 5201 by an A/D converter 5002 and supplied to a vertical filter 5006 for filtering signals in the vertical scanning direction (hereinunder referred to merely as "vertical direction") and a first 1-line delay circuit 5003.
The output 5202 of the first 1-line delay circuit 5003 is input to a second 1-line delay circuit 5004, a delay compensating circuit 5005 and the vertical filter 5006. The output 5203 further delayed by one line by the second 1-line delay circuit 5004 is input to the vertical filter 5006.
The vertical filter 5006 is generally composed of a filter which is called a 2-line comb filter, and the output of the vertical filter 5006 is input to a band-pass filter 5007.
The output 5205 of the band-pass filter 5007 is led out of an output terminal 5008 as a color signal and input to one input terminal of a subtracter 5009. The output 5206 of the delay compensating circuit 5005 is input to the other input terminal of the subtracter 5009. The delay compensating circuit 5005 is a circuit for compensating for the delay in the band-pass filter 5007. A brightness signal 5207 is output from the subtracter 5009 and led out of an output terminal 5010.
The operation of the conventional color filter with respect to a composite color TV of an NTSC system will now be explained.
The composite color TV signals 5201 which are synchronously sampled at a sampling frequency of f.sub.s =4.multidot.f.sub.sc (f.sub.sc represents a color subcarrier frequency) by the A/D converter 5002 are arranged two-dimensionally on a screen, as shown in FIG. 13.
Since f.sub.sc =(455/2)fH (fH represents a horizontal scanning frequency), 4 samples of color signals C having a phase difference of 180.degree. are extracted per period on each line.
In FIG. 13, the symbol Y represents a brightness signal, and C1 and C2 color signals. The blank circle is Y+Cl, hatched circle is Y-C1, white triangle is Y+C2 and hatched triangle is Y-C2.
If the delay of 1 sample and the delay of one line are expressed by Z.sup.-1 and Z.sup.-l, respectively, by Z conversion, EQU Z.sup.-1 =exp(-j2.pi.f/4f.sub.sc).
Since f.sub.sc 32 (455/2)fH, l=910.
The vertical filter 5006 extracts a line support signal 5204 including a color signal C for supporting a line which includes a color signal from the current input signal, the 1-line delayed signal 5202 and the 2-line delayed signal 5203. In this case, the transfer function HV(Z) of the vertical filter 5006 is EQU Hv(Z)=(-1/4).multidot.(1-Z.sup.-l).sup.2.
In other words, the line support signal HC(m, n) 5204 at the coordinates (m, n) on the screen shown in FIG. 13 is extracted in the form of EQU HC(m, n)=-(1/4){S(m, n-1)-2S(m, n)+S(m, n+1)}.
The line support signal 5204 also includes a brightness signal Y. The band-pass filter 5007 separates the color signal C(m, n), which is a high-frequency component, from the line support signal HC(m, n) 5204. The thus-obtained color signal 5205 is supplied to the subtracter 5009. The subtracter 5009 subtracts the color signal C(m, n) 5205 from a signal S (m, n) 5206 obtained by delaying the 1-line delay signal 5202 by the delay compensating circuit 5005 in correspondence with the band-pass filter 5007, thereby separating the brightness signal Y(m, n) as represented by the following equation: EQU Y (m, n)=S (m, n)-C (m, n).
In this case, the transfer function Hh(Z) of the band-pass filter 5007 is constituted, for example, as represented by the following equation: EQU Hh(Z)=(-1/32) (1-Z.sup.-2).sup.2 (1+Z.sup.-4).sup.2 (1+Z.sup.-8).
As described above, in the conventional brightness signal/color signal separating filter, the characteristics of the vertical filter and a horizontal filter for filtering signals in the horizontal scanning direction (hereinunder referred to merely as "horizontal direction") are combined in a fixed state. In other words, the brightness signal Y and the color signal C are separated by the band-pass filter both in the vertical and horizontal scanning directions.
Therefore, in an area in which the brightness and the color of a picture rapidly change, each of the brightness signal Y and the color signal C leaks out to the other's channel, which produces a deterioration in quality of the reproduced picture due to dot interference or the like.